1. Field of the Invention
The present invention relates to the combustion of materials, specifically to the high temperature combustion of metals or other energetic fuel.
2. Description of Related Art
The incineration of radioactive, chemical and mixed hazardous materials requires high temperature combustors. In addition to high temperature, the incinerator needs to produce clean combustion. Aluminum, for example, is a very energetic metallic fuel and may produce an adiabatic reaction temperature of up to 10,600° C. when it reacts with oxygen. The combustion product, aluminum oxide, is a valuable product for many industrial applications. The exhaust gas from aluminum combustion, for example, is clean and does not contain any unburned hydrocarbons, nitric or carbon oxides, or volatile organic components as pollution from the combustion of hydrocarbons. The combustion of aluminum in a water phase environment has a high temperature exhaust stream of aluminum oxide in gaseous and solid phases and water vapor. The lack of pollution from an aluminum combustion process makes an aluminum combustor ideal for the incineration of hazardous materials. The ability to use air or water as the oxidizer for combustion is another very attractive feature of the combustion of aluminum. Aluminum fuel could be used in underwater incineration and also for the propulsion of various underwater mechanical devices such as torpedoes or submarines.
Aluminum reacts with oxygen exothermically following the reaction:2Al(s)+ 3/2O2<->Al2O3(g)  (A)
The calorimetric heat of reaction at 1 Bar is (ΔH° (298K)=−404 kcal/mol). There is, however, a problem with the direct burning of pure aluminum following this reaction. In an oxygen environment (e.g., air), a strong layer of Al2O3 coats an aluminum particle. The particle temperature must be raised past the Al2O3 melting point (2027° C.) to obtain ignition. The ignition delay time includes the time needed to heat, and then melt, the Al2O3 layer, plus the diffusion time for the O2 to reach the aluminum surface and react. When Aluminum vaporizes, however, the kinetics is very rapid and no longer controls the ignition process. The sum of all these times is comparatively large, so particles escape with the high-speed gas flow from the combustion chamber without chemical reaction.
A reduction of the ignition delay time and a lowering of the ignition temperature, of aluminum is possible by oxidizing aluminum with steam in the following the reaction:2Al(s)+3H2O<->Al2O3(g)+3H2  (B)
The calorimetric heat of reaction at 1 Bar is (ΔH°(298K)=−230 kcal/mol). In a steam atmosphere, a hydroxide layer that is less protective than aluminum oxide covers the aluminum particles. According to reaction (B), the ignition temperature drops to 1323-1423° C. shortening the ignition time. This reaction is most attractive for underwater power generation and propulsion, since it does not require air. The oxidizer, seawater, is provided directly by the environment, dramatically reducing on-board storage requirements. The power density of reaction (B) is less than reaction (A) since the heat of reaction, for the same 2 moles of aluminum, is less for reaction (B).
Due to the high temperature of reaction (B), however, part of the water dissociates, resulting in a release of oxygen. This oxygen is used to burn the H2 produced by reaction (B) in the following reaction:3H2+ 3/2O2(g)<->3H2O(g) (ΔH° (298K) at 1 Bar=−174 kcal/mol).  (C)
Reactions (B) and (C) together produce the same energy release as (A) with less restrictive ignition constraints. Reaction (C) does not, however, need any external ignition source, since the H2 produced in reaction (B) will be above its auto ignition temperature in an O2 atmosphere.
Since the destruction of the aluminum oxide layer, by melting, is critical to aluminum combustion, the rapid heating of the particles is especially important. Heating can be produced by friction in a strongly turbulent stream flow or by convective/conductive heat transfer. In addition, sufficiently intensive friction (shear) forces or head-on collisions between particles could crack the oxide or hydroxide layer protecting the pure aluminum and ignite it. These have been the traditional mechanisms of heating aluminum prior to combustion. Through estimation of all above effects, Applicants have recognized that neither heating nor collisions effectively destroy the aluminum oxide layer. This indicates that combustor designs based on these mechanisms will not work efficiently.
Accordingly, recognized is the need for a combustion system including a fuel supply apparatus which can supply a metal fuel to a combustion apparatus having a relatively thin oxide layer and/or remove at least portions of the oxide layer immediately prior to consumption by the combustion apparatus without requiring a direct heating application. Also recognized is the need for a combustion system including a combustion apparatus capable of receiving such energetic metal fuel and to provide a highly efficient combustion of such energetic metal fuel that operate efficiently in either an air or water environment.